Gravity According to Galilei, Newton, Einstein and Mach

Abstract

As mentioned in Chap. 1, Galileo Galilei (1546–1642, Fig. 7.1) discovered that without visual information it is impossible to distinguish whether we are at rest or moving with a constant speed. Earth moves with a speed of about 30 km/s in its orbit around the sun, but we feel nothing of this motion. It appears to us that Earth is completely at rest. Galilei discovered that the only thing we can feel is a change in velocity: when a car accelerates, we are pressed against the back of our seats, and if it suddenly brakes, we shoot forward. The reason for these effects is that if one does not exert any force on an object, it wants to persist in moving on with the same speed. (This makes you shoot forward when your vehicle is braked.) This is Galilei’s law of inertia. It was Newton’s discovery that, in order to change the velocity of an object, a force is needed. To make a car accelerate, we need an engine that causes the speed of a car to increase. In order to make the speed decrease, we must hit the brakes, such that a decelerating force is exerted on the wheels. Newton (1643–1727, Fig. 7.2) has expressed this relation between force and acceleration in his famous law of inertia.

Discovering is seeing what everybody has seen and thinking what nobody has thought

Albert Szent Gyorgyi (1893–1986), American scientist